1. Field of the Invention
The invention relates to multi layer coatings which use interference effects rather than absorptive dyes to modulate spectral reflectances. The coatings may be in the form of a thin film or a pigment and may be applied to products or articles for anticounterfeiting purposes in order to identify genuine goods. The coatings may also be used for spectral control of thermal emittance or for thermal control purposes.
2. Discussion of Prior Art
The use of pigments which use interference effects to provide colour is becoming increasingly popular in many areas. Absorptive colour pigments are often based on toxic heavy metals which can be problematic. Interference colours have the further advantage that they may be angle tuned to provide additional decorative effects. Furthermore, once the material system has been qualified, new colours may be produced without the need for extensive weathering trials.
Optically variable pigments (OVPs) based on interference effects and having a metal-dielectric-metal layer structure (Mxe2x80x2-D-M) are known in the prior art (U.S. Pat. No. 4,705,356). The structure comprises a reflective metal layer (Mxe2x80x2) a dielectric layer (D) and a thin absorbing metal layer (M), which forms a Fabry Perot cavity. The OVP has a large colour shift with viewing angle which makes it difficult for counterfeiters to reproduce by other means. Structures based on multiple periods of dielectric spacer-metal absorber layer pairs and constructed on reflective metal layers (e.g. Mxe2x80x2(D)n) are also known (U.S. Pat. No. 5,214,530). The structures are peak suppressing (i.e. reflection minima are suppressed) for the purposes of producing stronger chromatic effects.
Relevant background to the present invention can also be found in U.S. Pat. No. 5,437,931 which relates to optically variable multi layer films providing reflection characteristics in the visible wavelength region.
The present invention specifically relates to multi layer interference coatings which have strong reflectance characteristics in the infrared wavelength region. The structures are peak suppressing but have the advantage over known multi layer structures that they comprise fewer layers making fabrication more straightforward. Furthermore, in one form of the invention, fabrication is not only easier in this respect but also because of the particular materials used.
There are a number of applications for coatings exhibiting strong reflectance characteristics in the infrared wavelength region. In particular, for covert marking and anticounterfeiting applications, hidden spectral features may be used to uniquely identify an article or product. Conventional multi layer interference structures, however, do not operate well in the infrared wavelength region. The coatings may be constructed such that they have strong reflectance characteristics in the infrared.
The invention also relates to an anticounterfeiting or product tracking system in which the multi layer coatings may be incorporated, the operation of which may be covert.
According to the present invention, a multi layer interference coating, having a reflectance spectrum in the infrared wavelength region comprising at least one maximum, comprises;
a reflective layer having at least one surface for carrying one or more multi layer stacks,
wherein each multi layer stack comprises a first layer of dielectric material, a layer of absorbing material and a second layer of dielectric material arranged in series with the layer of absorbing material situated between the first and second layers of dielectric material,
wherein the second layer of dielectric material has substantially the same optical thickness as the first layer of dielectric material at a wavelength substantially corresponding to a maximum in the reflectance spectrum and wherein the layer of absorbing material has a refractive index n and an optical constant k,
such that incident electromagnetic radiation, having a wavelength at which odd multiples of half wavelengths substantially correspond to the optical thickness of the coating at said wavelength, is substantially absorbed within the coating.
Preferably, the first dielectric material is the same as the second dielectric material. At least one of the first or second dielectric materials may be any one of titanium oxide (TiO2), magnesium fluoride (MgF2), zinc sulphide (ZnS), zinc selenide (ZnSe), silicon (Si), germanium (Ge) or barium fluoride (BaF2)
In a preferred embodiment, the n/k ratio of the absorbing material is between 0.7 and 1.3, and is preferably substantially equal to 1, in the infrared wavelength region.
The reflective layer may be a metal, for example, gold, silver or aluminium. The absorbing material may be a metal, for example, chrome (Cr), vanadium (V), palladium (Pd), nickel (Ni) or platinum (Pt).
In another embodiment of the invention, the absorbing material may be a substoichiometric metal oxide. Preferably, the substoichiometric metal oxide may be of the same material as the layer of dielectric material. For example, the substoichiometric metal oxide may be titanium oxide (TiOx) and the dielectric material may be titanium dioxide (TiO2).
In another form of the invention, the absorbing material may be a conducting oxide, a conducting nitride or a conducting silicide, for which the n/k ratio is substantially equal to 1 in the infrared wavelength region. For example, the absorbing material may be indium tin oxide (ITO), doped tin oxide, for example SnO2:F, or titanium nitrate, (TiN). Alternatively, the absorbing material may be vanadium dioxide (VO2) substoichiometric vanadium oxide (VO2xe2x88x92x) or doped VO2 and the reflectance spectrum of the coating may be varied with temperature.
If a conducting oxide, nitride, silicide or sulphide is used as the absorbing material, it may be advantageous to use a like conducting oxide, nitride or silicide as the reflective substrate. Alternatively, the reflective layer may be a metal, such as gold, silver or aluminium.
The reflective layer may have two opposite facing surfaces wherein at least one multi layer stack is deposited on each of the two opposite facing surfaces such that the coating has a substantially symmetric structure about the reflective layer.
The reflective layer may comprise a reflective material deposited on a non-reflecting particulate substrate or may be a reflecting particulate substrate.
The coating may be in the form of a thin film which may be flaked into fragments and incorporated into a paint or ink. Alternatively, the reflective layer may be substantially spherical, with at least one multi layer stack is deposited on the substantially spherical reflective layer. The substantially spherical multi layer structure may then be incorporated into a paint or ink.
The coating may be applied directly to the surface of an article or applied to a label to be applied to an article. Alternatively, the coating may be incorporated into a moulded article.
In another embodiment of the invention, where the absorbing material is a non-metal material, the second layer of dielectric material may be absent in at least one of the multi layer stacks,
such that incident electromagnetic radiation, having a wavelength at which odd multiples of quarter wavelengths substantially correspond to the optical thickness of the coating at said wavelength, is substantially absorbed within the coating.
In this form, the absorbing material may be a conducting oxide, nitride, silicide or sulphide, for example ITO, doped tin oxide (for example SnO2:F), TiN, VO2, substoichiometric VO2 (VO2xe2x88x92x) or doped VO2. Alternatively, the absorbing material may be a substoichiometric metal oxide, for example TiOx.
According to another aspect of the invention, a system for marking an article and checking its authenticity comprises;
a multi layer interference coating, having a reflectance spectrum comprising at least one maximum, wherein the coating is applied to the article to be authenticated,
means for illuminating the coating with incident radiation comprising one or more wavelengths wherein one or more of the wavelengths substantially correspond to a maximum or a minimum in the reflectance spectrum of the coating and
means for detecting radiation reflected from the coating at substantially one or more of the wavelengths,
whereby the detection of the reflected radiation provides an indication of the authenticity of the article.
A comparison of the reflected radiation at two or more wavelengths may provide an indication of the authenticity of the article.
According to another aspect of the invention, a system for covertly marking an article and checking its authenticity comprises;
a multi layer interference coating having a temperature dependent reflectance spectrum, wherein the coating is applied to the article to be authenticated,
means for varying the temperature of the coating such that the reflectance of the coating at one or more wavelengths may be varied as the temperature is varied,
means for illuminating the coating with infrared radiation comprising one or more wavelengths substantially corresponding to one or more of the wavelengths at which the reflectance varies and
means for detecting infrared radiation reflected from the coating at one or more of the wavelengths at which the reflectance varies,
whereby a comparison of the reflected radiation before and after the temperature of the coating is varied provides an indication of the authenticity of the article.
A single laser may be used to both illuminate and vary the temperature of the coating. A thermal imager or a spectrophotometer may be used to detect the reflected radiation.